1. Field of the Invention
The present invention relates to an opening and closing apparatus for a manhole cover for safely and securely opening, closing, and moving a special manhole cover such as a thick-walled large-diameter cover, a double cover and a raised cover, as well as a normal manhole cover.
2. State of the Prior Art
A manhole cover having a diameter of 1 m or more and a weight of 300 to 400 kg was mainly used in the past. Such a manhole cover was not bonded to a seat for maintaining the manhole cover at rest, and a gap of 5 mm or more was defined between the manhole cover and a side wall of the seat. In opening the manhole cover, a crowbar was used to pry the manhole cover, or a hook was used to catch hold of the manhole cover and raise it forcefully, then sliding it horizontally. However, as the manhole cover was a heavy load, physical fatigue, injury, acute lower back pain, etc. of the operator become a problem.
Subsequently, the facts that the manhole cover could be relatively easily opened by anyone, and that the manhole cover could be undesirably opened by sewage flowing back in case of flood, were considered as other problems, in view of both security and safety. Accordingly, the manhole cover was bonded to the seat by adhesive to prevent easy opening of the manhole cover. Further, the gap between the manhole cover and the side wall of the seat was also zero, so that even when someone tried to pry the manhole cover up with a lever, the lever was hindered by the side wall of the seat so that the manhole cover could not be completely opened.
In these circumstances, the present applicant filed a utility model entitled "Opening and Closing Machine for Manhole Cover" on Jul. 24, 1990 (Japanese Utility Model Application No. Hei 2-77849). The utility model discloses an opening and closing machine for a manhole cover, comprising a hydraulic pump adapted to be vertically expanded and contracted, a horizontal extension bar extending horizontally from a side surface of the hydraulic pump at an upper portion thereof, wheels provided at a lower portion of the hydraulic pump arranged in tandem in a direction perpendicular to the horizontal extension bar, a supporting portion projecting downward from one end of the horizontal extension bar, and a tightening rod with a lower hook extending vertically through the horizontal extension bar at an intermediate region thereof. Further, this utility model describes at page 3, line 12 to page 4 line 6 a manner of usage of the opening and closing machine as follows. Two catch holes of every manhole cover, inclusive of a round cover and a rectangular cover, are formed at right and left positions such that the raised manhole cover can be balanced in weight. The opening and closing machine is located on the right-hand side of the manhole cover, and another opening and closing machine, having the same structure as that of the above machine, is located on the left-hand side of the manhole cover in such a manner that the sets of wheels of both of the machines are opposed to each other in parallel. The wheels are laid on the ground surface, and the supporting portions are laid on the manhole cover. The lower hooks of the tightening rods vertically extending through the horizontal extension bars are brought into engagement with the catch holes of the manhole cover, and then tightened. Thereafter, the hydraulic pumps of the two opening and closing machines are simultaneously expanded. As a result, the manhole cover is raised in a horizontal condition by strong forces generated from the hydraulic pumps. When the manhole cover is horizontally pulled in this raised condition, the wheels function to easily move the two machines holding the manhole cover, thus completely opening the manhole cover. The above opening operation is completed within one minute. In closing the manhole cover, the above operation is performed in the reverse order.